Electrostatic repelling cylinders for filament flyback control

ABSTRACT

Electrostatically charged groups of continuous filaments entrained in a plurality of gaseous streams are forwarded by means of jet devices toward a receiving area. The tendency of filaments at the edges of each group to escape from its group and to become attracted to nearby grounded surfaces is reduced by forming an electrostatic field of the same polarity as the charge on the filaments in the zone between the jet devices and the receiving area.

Elite Sttes ate axter et [151 3,655,305 [4 1 Apr. 11, 1972 [54] ELECTROSTATIC REPELLWG CYLINDERS FOR FHLAMENT FLYBACK CONTROL [72] Inventors: James Fletcher Baxter; Samir Costandi Debbas, both of l-lendersonville, Tenn.

[73] Assignee: E. I. du Pont de Nemours and Company,

Wilmington, Del.

[22] Filed: Jan. 26, 1970 [21] App1.No.: 5,731

[51] Int. Cl. ..D0ld 7/00, B65h 53/00 [58] Field oiSeareh 18/8 264/177 R, 177 F [56] References Cited UNITED STATES PATENTS 3,156,752 11/1964 Cope ..18/8 NBX 2,123,992 7/1938 Formhals ..18/8 EX 2,160,962 6/1939 Formhals ..18/8 E X 2,336,746 12/1943 Manning ..264/D1G. 75

2,349,950 5/1944 Fonnhals ..18/8 E X 2,158,415 5/1939 Formhals ..18/8 E X 2,168,027 8/1939 Gladding ..18/8 E 2,187,306 1/1940 Formhals ..18/8 E X 3,325,906 6/1967 Franke ..18/8 B X Primary Examiner-Robert D. Baldwin Assistant Examiner-Michael 0. Sutton Attorney-Sol Schwartz [57] ABSTRACT Electrostaticaily charged groups of continuous filaments entrained in a plurality of gaseous streams are forwarded by means of jet devices toward a receiving area. The tendency of filaments at the edges of each group to escape from its group and to become attracted to nearby grounded surfaces is reduced by forming an electrostatic field of the same polarity as the charge on the filaments in the zone between the jet devices and the receiving area.

2 Claims, 3 Drawing Figures ELECTROSTATIC REPELLING CYLINDERS FOR FILAMENT FLYBACK CONTROL BACKGROUND OF THE INVENTION The present invention relates to a process for controlling groups of electrostatically charged continuous filaments moving in a gaseous stream. More particularly, it relates to methods for obtaining continuous-filament nonwovens having improved uniformity.

Processes for the preparation of continuous-filament nonwoven fabrics utilizing electrostatic charging of the filaments to separate and distribute the filaments on a laydown receiver are known in the art. For example, U.S. Pat. No. 3,338,992, describes a process wherein a multifilament strand of continuous filaments under tension is electrostatically charged by known techniques, for example by passing the filament through a corona discharge zone. The charged filaments are then forwarded by means of a jet device toward a web laydown zone, the tension on the filaments being released shortly after the exit of the jet device thereby permitting them to separate due to the repelling effect of the applied electrostatic charge. The filaments, while thus separated, are collected as a nonwoven web.

The preparation of commercially desirable wide webs requires the blending of the output of a plurality of jet devices in such a process. A blending process to form wide webs is described in US. Pat. No. 3,402,227 wherein the jet devices are so spaced to provide 50 to 80 percent overlap of the areas of initial filament deposition on a moving foraminous receiver covering a suction area, while withdrawing air at a rate at least five times as great as the flow of air from the jet devices. Slot jet devices of the type described in US Pat. No. 3,302,237, fitted with a diffuser section of the type disclosed in US. Pat. No. 3,325,906, are particularly suited in the preparation these nonwoven webs.

In the production of continuous filament nonwovens by the aforementioned procedures, using electrostatically charged filaments, it has been found that unit weight uniformity of the sheet improves with increase in fiber charge. At very high charge levels, however, it has been noted that filaments occasionally escape from the air stream at the edges of each group of filaments which is moving toward the laydown receiver. These escaping filaments (flybacks) become attracted to any grounded surface and may cause laydown imperfections. The problems become magnified at increased throughputs due to increased number of filaments per jet and/or increased jet air flows which cause turbulence. The present invention provides a process and apparatus for the production of continuous filament nonwoven sheets which permits higher electrostatic charges to be used on the filaments before substantial flyback occurs.

SUMMARY OF THE INVENTION In the instant process for obtaining continuous filament nonwoven webs of improved uniformity, electrostatically charged groups of continuous filaments entrained in a plurality of gaseous streams are forwarded by means of jet devices toward a receiving area. Filament flybacks are prevented or reduced by forming an electrostatic field of the same polarity as the charge on the filaments in the zone between the jet devices and the receiver surface. This is achieved by positioning at least one electrostatically charged cylinder between the jet device and the laydown receiver in the vicinity of the filaments being forwarded toward the receiver and substantially parallel to the short axis of the cross section of the jet it controls. The distance between the cylinder and the center line of the jet (in the plane substantially perpendicular to the receiver) should be less than the distance between the edge of the swath at the receiver surface and said center line. The cylinder should be in a plane substantially parallel to and at a point above the laydown receiver that is at least 30 percent and preferably between 3070 percent of the distance between the bottom of the jet device and the receiver. The

LII

distance between the cylinder and the receiver measured in inches is represented by H in the formula which follows. When the filaments have an electrostatic charge Q measured in microcoulombs/meter the voltage V in volts applied to the cylinder should be chosen such that WHO is 200 or greater.

The preferred embodiment of the invention contemplates the use of slot jet devices (i.e., having rectangular cross sections) of the type described in US. Pat. No. 3,302,237, fitted with diffuser sections of rectangular cross section such as described in US. Pat. No. 3,325,906. Two electrostatically charged cylinders are used, positioned between the bottom of the diffuser section and the laydown receiver, the cylinders being parallel to the narrow side walls of the diffusers and being distant about 1 to 2 inches from the edge filaments of the group being forwarded to the receiver. The height of each cylinder above the laydown receiver is 30-70 percent of the distance between the receiver and the bottom of the diffuser and the voltage applied to the cylinders is chosen so that in each case, the ratio WHO is greater than about 200.

Electrostatically charged screens or plates may be used to repel the filaments and maintain them in the air stream, however, cylinders are preferred. The main reason is that with sharp edged devices only a limited charge can be obtained before corona discharge occurs and this has a disrupting effect on the charge on the filaments. When cylinders are used, it is possible to obtain very high voltages before any corona discharge occurs and the electrostatic field generated can then be sufficiently high to produce the desired result. The diameter of the cylinders should be at least one-half inch with a preferred range of from one to three inches in diameter. The cylinders should be made from a highly electrically conductive material. Aluminum has been found to be suitable.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic representation of an apparatus for spinning two types of filaments and applying an electrostatic charge prior to the formation of a nonwoven web. The jet device including its diffuser portion is viewed from its narrow side.

FIG. 2 is an enlarged view of the exit of a diffuser showing the position of the electrostatic cylinders of the invention. A portion of the wide side of the jet diffuser is shown.

FIG. 3 is a plan view representation of an apparatus suitable for producing wide nonwoven webs by blending the output from several spinning positions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The process of the present invention is preferably applied to the production of continuous-filament nonwovens composed of two types of filaments: higher-melting matrix filaments constituting at least 75 percent by weight of the total structure and lower-melting binder filaments constituting the remainder. The two types of filaments are preferably melt spun and blended with one another prior to deposition on the laydown receiver.

Referring to FIG. 1, matrix filaments 1 are spun from spinneret 2 and quenched by radial quench diffuser 3. The filaments are brought together in ribbon form on concave guide 4 which, in combination with cylindrical guide roll 6 and concave guides 5 and 8, converges the ribbon of filaments to the width required for passage through jet device 10. Draw rolls 9 provide the tension to attenuate the filaments in the region between the spinneret and concave guide 4. This process of spin drawing is taught by Hebeler in US. Pat. No. 2,604,689, and is especially useful when a nonwoven web with matrix filaments of polyethylene terephthalate is being prepared. The filaments are electrostatically charged by means of corona discharge devices comprising target bars 11 and charging heads 7, as described in DiSabato and Owens US. Pat. No. 3,163,753.

Binder filaments 12 (generally fewer in number) are spun from spinneret 13 and quenched by cross flow air using chimney 14. The filaments pass through a fan-shaped comb 15 and contact the cylindrical guide 16. Relative movement of the guide 16 with respect to comb 15 changes the depth of the filaments within the fan-shaped comb and alters the width of the ribbon accordingly. The correct width is obtained when the edge filaments contact the pins 20 of a ribbon width gauge 17. An electrostatic charge is applied using the corona device and grounded target bar and the width of the ribbon is stabilized by passage over a grooved guide 18. The ribbon of binder filaments is diverged to the same width as the ribbon of matrix filaments leaving the concave guide 8 and the two ribbons are brought together and uniformly blended on the first draw roll 9. Binder filaments of a 79/21 copolymer of polyethylene terephthalate/polyethylene isophthalate are suitable for use with polyethylene terephthalate matrix filaments.

As in the case of the matrix filaments 1,the tension required to attenuate the binder filaments and to move them over the guide and target bar of the corona discharge device is provided by the draw rolls 9. Jet device provides the tension needed to strip the composite filament ribbon from the last draw roll and to forward the filaments toward a web laydown zone of a foraminous belt receiver 23.

FIG. 2 shows the relative position of the bottom of the diffuser 10, the filaments 22, two electrostatically charged cylinders 21 and the width of laydown at the receiver 23. When a diffuser is used, the cylinders are positioned at 30-70 percent of the distance from the bottom of the diffuser to the laydown receiver. It is also noted that the cylinders need not both be the same distance from the receiver, nor need they be at the same distance from the jet center line, provided that the distance of each cylinder from the center line is less than the distance from the edge of the swath laid down at the receiver to said center line.

In FIG. 3, filaments from jet-diffuser assemblies 10 are forwarded and deposited onto a moving foraminous receiver 23 in a suction laydown zone defined by suction trough 24 which is arranged diagonally across the receiver 23. The areas 25 represent the patterns in which the filaments contact the receiver. Electrostatically charged cylinders 21 are shown positioned parallel to and on either side of the row of jets to prevent filament fiyback and to improve the uniformity of laydown of the nonwoven web. Each cylinder is connected to a separate high DC voltage power pack capable of supplying up to 100 kilovolts of the same potential as that on the filaments. A series resistor of about 250 megohm is placed between the cylinder and cable to eliminate power pack and cable capacitance effects and reduce short circuit current. The cylinders, which are preferably made of aluminum, are mounted on insulated brackets which allow vertical and horizontal motion relative to the diffuser exit so that they may be placed in the optimum position. The resulting nonwoven web (not shown) moves away from the suction laydown zone for further processing.

The high electrostatic field generated by the cylinders directs the filaments down toward the laydown receiver thus preventing flyback. Accordingly, higher charges can be placed on the filaments to achieve improved uniformity. The field strength from the cylinders can be increased or reduced as a function of the voltage. At higher filament charge levels, higher field strength is required to prevent fiybacks and constrain the swath width to the required dimensions suitable for good blending. The geometrical shape of the cylinders due to their smooth curvature, permits the use of very high voltages (80-100 KV) without arcing to the nearest ground. Screens for instance would create an arcing problem around 30 KV due to the sharp knuckles present, around which current density concentrates. Therefore screens or plates do not have as good a constraining effect on the filaments and they restrict access to the bottom of the jets.

In the following examples, the charge on the filaments was measured using a Faraday Cage. The charge, in microcoulombs/meter is calculated from the measured voltage, the capacitance of the system and the surface area of the fibers (Average of 10 highest peaks- Average of 10 lowest minimal 11 1/2 (Average 10 highest-l-Average 10 lowest) Extreme percent Range= (Highest peak Lowest minimum) 100 1/2 (Highest peak+ lowest minimum) 10 readings based on 40" sample width (wider samples require porportionally more readings).

EXAMPLEI Poly(ethylene terephthalate)matrix filaments and poly( ethylene terephthalate/isophthalate) (79/21 copolyester binder filaments are melt-spun at 280 C. through multihole spinnerets, at the rate of 1.2 and 0.94 gmjminutelhole respectively and a ratio of 5.5:1 matrix to binder filaments. The apparatus used was similar to that schematically represented in FIG. 1 and in FIG. 3 where a plurality of positions are schematically represented. The filaments were quenched, charged and passed around draw rolls operating at 3,000 ydjmin. before entering a slot jet for stripping and for warding to the laydown conveyor belt. The slot jet of the type described in Cope et a1. U.S. Pat. No. 3,302,237, was fitted with a diffuser section as described in Franke US. Pat. No. 3,325,906. the distance between the bottom of the diffuser and the laydown receiver was 19.75 inches resulting in a swath about 20 inches wide at the receiver.

Tests were run on a single position using an 18 inch long aluminum pipe with a diameter of 1.5 inches located on one side of the group of filaments being forwarded toward the receiver. In the absence of the pipe, the maximum charge which could be placed on the filaments before flyback occurred was 11.6 microcoulombs/meter With the cylinder located 10 inches above the laydown receiver and 8 inches away from the center line of the jet, the filament charge could be raised to about 14.3 microcoulombs/meter with the cylinder operating at 32 kilovolts. This corresponds to a ratio of WHO of about 220.

EXAMPLE 2 Using the same spinning conditions as in Example 1, the test was repeated except that a longer cylinder was used which covered six spinning positions. The cylinder was positioned 10.25 inches from the laydown receiver and 8 inches from the jet center lines. Tests were run to determine the maximum charge which could be placed on the filaments at each of the six positions, before flyback occurred, both in the absence and the presence of the cylinder charged to a voltage of 34 KV. The results shown in Table I indicate that the maximum charge attainable without fiyback was increased on all positions in the presence of the electrostatically charged cylinder.

EXAMPLE 3 A nonwoven material was produced with the apparatus and process conditions described in Example 1, the speed of the laydown receiver being adjusted to make a product with a weight of 1.5 oz./yd Material was collected (a) as a control with no electrostatic repelling cylinders; (b) with a 1.5 inch diameter cylinder positioned on the left side (looking in the direction of movement of the receiver as shown by arrow 26 in FIG. 3) along the entire length of the spinning positions, 10.25 inches from the laydown receiver and 8 inches from the center line of the jets; and (c) with an additional cylinder placed on the right side of the jets 12.25 inches from the laydown receiver and 8 inches from the center line of the jets.

The experimental conditions and results obtained are given in Table II and indicate that a significant increase in filament charge level was obtained when one charged cylinder was used resulting in a significant unifonnity improvement. The use of two cylinders allowed further increase in the filament charge level and was accompanied by a further improvement in the uniformity of the sheet.

extreme lombs/m ls rs ls rs" Control One cylinder 14.8/24 15 32 208 Two cylinders 14.4/24 16.4 32 52 208 259 left side right side What is claimed is:

1. In an apparatus for preparing a nonwoven web from a plurality of continuous filaments which includes spinneret means for providing a group of continuous filaments, means for forming the group into a ribbon of filaments, charging means for the application of an electrostatic charge to the ribbon of filaments, a slot jet device for forwarding and directing the filaments toward a laydown zone on a receiver and a receiver for collecting the filaments as a nonwoven web in said zone, the improvement comprising an electrically conductive cylinder of at least inch in diameter suitably connected to a DC voltage supply and lying in a plane parallel to the receiver, said cylinder being located at a height above the receiver surface that is at least 30 percent but no more than percent of the distance between said receiver and said jet exit and so positioned that it lies between the center line of said jet and the edge of the laydown zone and is substantially parallel to the short axis of the jet cross section to prevent filaments at the narrow edge of the ribbon from escaping from the body of filaments and becoming attracted to nearby grounded surfaces other than the receiver surface.

2. The apparatus of claim 1 wherein one electrically conductive cylinder is positioned on each side of the center line of the jet. 

1. In an apparatus for preparing a nonwoven web from a plurality of continuous filaments which includes spinneret means for providing a group of continuous filaments, means for forming the group into a ribbon of filaments, charging means for the application of an electrostatic charge to the ribbon of filaments, a slot jet device for forwarding and directing the filaments toward a laydown zone on a receiver and a receiver for collecting the filaments as a nonwoven web in said zone, the improvement comprising an electrically conductive cylinder of at least 1/2 inch in diameter suitably connected to a DC voltage supply and lying in a plane parallel to the receiver, said cylinder being located at a height above the receiver surface that is at least 30 percent but no more than 70 percent of the distance between said receiver and said jet exit and so positioned that it lies between the center line Of said jet and the edge of the laydown zone and is substantially parallel to the short axis of the jet cross section to prevent filaments at the narrow edge of the ribbon from escaping from the body of filaments and becoming attracted to nearby grounded surfaces other than the receiver surface.
 2. The apparatus of claim 1 wherein one electrically conductive cylinder is positioned on each side of the center line of the jet. 